The clinical impact of IHIV-1 variant evolution and the special cases of dual- and super-infection have significant implications for understanding AIDS pathogenesis, virus transmission, and vaccine development. Dual infection can occur as a result of co-infection, wherein acquisition of two HIV-1 viral strains occurs during primary infection, or superinfection, wherein a second viral strain infects sometime after primary infection. Both forms of transmission are now well-documented, although the timing and frequency of each, and their impact on disease progression remain poorly understood. Our recent studies of a small cohort of seroconcordant sexual partners suggest that superinfection is extremely frequent. We propose in this continuing application to follow up our preliminary findings, expanding our studies of the virology of HIV-1 Infection and the impact of host genotype on disease outcome, Including exploration of the host genes we have recently implicated in contributing to the rate of disease progression. In addition, we are broadening our studies to examine the differential impact ofthe humoral and cellular immune responses on viral evolution, immunological escape, and dual infection. Specifically, we propose to address the following specific aims and linked hypotheses using genetic, virologic, immunological and bioinformatics methodologies: Aim la-b) Determine the frequency of primary versus superinfection, and of X4 variant evolution during the progression to AIDS in the MACS. Aim Ic) Assess anti HIV-1 neutralizing Ab and CTL responses In individuals with mono and dual HIV-1 infection. Aim 2) Determine the evolutionary and functional changes in Env that accompany transmission as a function of viral genetic subtype, route of transmission and gender of the recipient. Aim 3) Biologically validate and investigate our set of novel AIDS restriction genes. RELEVANCE (See instructions): The proposed studies are a continuation of our efforts to define the molecular events leading up to the development of AIDS. The expected benefits from our research include: 1) increasing our basic understanding of the infection and disease process, 2) developing new tools for optimizing the timing of antiretroviral therapy, and 3) understanding ofthe requirements for blocking reinfection, and 4) evaluation of the relative role of cellular and humoral immune responses in driving immunological escape in HIV during infection.